A conventional glassmelting furnace employs air-fired burners to melt the glass forming materials. Because of the very high temperatures required to melt the glass forming materials, a glassmelting furnace operates at temperatures which are among the highest of all industrial furnaces. These high operating temperatures cause a large amount of the heat produced by the burners to be lost up the flue. Typically only about 15 to 20 percent of the energy produced by the burners is available to the glassmelting furnace as useful heat when the burners are fired with ambient air and natural gas.
Those skilled in the art have addressed this problem by passing the flue gases from the glassmelting furnace in indirect heat exchange with incoming air to preheat the air and thus transfer heat, which otherwise would have escaped up the flue, back to the glassmelting furnace. These heat exchangers, known in the art as regenerators, significantly raise the efficiency of commercial air-fired glassmelting furnaces.
Nitrogen oxides, known as NO.sub.x, are considered to be detrimental environmental pollutants and the reduction of NO.sub.x emissions is an increasingly important goal. The conventional air-fired glassmelting process is a significant generator of NO.sub.x for two reasons. First, nearly 80 percent of air is nitrogen thus bringing significant amounts of nitrogen into the combustion process for the formation of NO.sub.x. Second, the high temperatures required for the glassmelting operation kinetically favor the formation of NO.sub.x.
It is known that the thermal efficiency of the glassmelting step can be increased by the use of oxygen or oxygen-enriched air as the oxidant. Moreover, the use of oxygen or oxygen-enriched air reduces the amount of nitrogen which is present in the combustion zone. However, the substitution of oxygen or oxygen-enriched air in place of air will cause the combustion reaction to proceed at a higher temperature which kinetically favors the formation of NO.sub.x. In addition, the cost of oxygen often makes glassmelting using oxygen as the oxidant more expensive than a conventional system using air as the oxidant, despite the increased thermal efficiency attainable with oxygen.
Accordingly, it is an object of this invention to provide a glassmelting method having high efficiency while achieving reduced NO.sub.x generation.